This invention relates to resistor spark plugs, more particularly to resistor glass seal spark plugs having a resistor between the lower end of the terminal screw (hereinafter referred to as "screw head") and the center electrode, said resistor comprising electrical conductive glass seals and a resistor glass seal placed therebetween, and said resistor spark plugs being effective for suppressing noise signals generated at the time of spark discharge.
Noise signals produced from automobiles jamming communication waves are mainly produced from spark plugs. In order to reduce such noise signals, it has been known that spark plugs having a built-in resistor are effective as disclosed in U.S. Pat. Nos. 3,567,658 and 3,577,355. In order to attain such an object, there have been known resistor glass seal plugs produced by forming a part of the center electrode of the spark plug by using a mixture of glass and electrical conductive powders so as to have a resistor built-in and by forming airtight sealing between the center electrode and the insulator centerbore. The composition of the electrical conductive glass seals at the side of the screw head and at the side of the center electrode of conventional resistor glass seal spark plugs is almost composed of an electrical conductive metal powders such as copper powder and borosilicate glass and the composition of the resistor glass seal sandwiched between the electrical conductive glass seals mentioned above is in general composed of carbon, silicon carbide, glass and a filler. But these conventional resistor glass seal spark plugs have disadvantages in that they have larger voltage coefficient of resistance (VCR) and are inferior in the amount of noise reduction. Causes for making the VCR larger have been studied and found to be that when airtight sealing of the center electrode is conducted by heating a glass seal material at high temperatures and hot pressing it, a high resistant layer can easily be formed at the contacting portion between the electrical conductive glass seal layer at the side of the screw head and the resistor glass seal layer due to the flow of the glass seal material and said high resistant layer has a larger shared voltage when a high voltage is applied thereto, and that since the electrical conductive glass seal at the side of the screw head intrudes into the resistor glass seal layer, the length of which is substantially shortened due to contiguity of the electric conductive glass seal at the side of the screw head to that at the side of the center electrode, and field strength per unit length applied to the resistor becomes larger. Further it has been found that when the amount of silicon carbide to be added to the resistor glass seal layer becomes larger, VCR becomes larger.